The Naked Scientists

The Naked Scientists Forum

Author Topic: How do aeroplane wings work?  (Read 5473 times)

Robert Bronson

  • Guest
How do aeroplane wings work?
« on: 27/03/2011 02:30:03 »
Robert Bronson  asked the Naked Scientists:
   
Hello, I have just found your podcasts this year and love your show

I have noticed, however, that on several occasions your team has described the flight of airplanes as being caused by the air pushing up on the wings.  

It was always explained to me that airplanes fly due to lift created by the vacuum caused on the top of the wing because of the curvature of the wing. 

I was told that it was similar physics to why sailboats work. Which is the correct answer?

Robert
Seattle, WA

What do you think?
« Last Edit: 27/03/2011 02:30:03 by _system »


 

Offline JMLCarter

  • Sr. Member
  • ****
  • Posts: 143
    • View Profile
How do aeroplane wings work?
« Reply #1 on: 27/03/2011 02:38:31 »
Both answers are correct, it depends on airspeed and angle of attack.

Flying using the wing surface to impact the air at high angle of attack creates a lot of drag and is not very efficient. However, it works well at low speed, whereas the curved wing pressure differential caused lift is more effective at higher speed and creates very little drag.

Early aircraft were basically kites with engine that used drag to get lift, the modern wing was more of a later development.
 

Offline Roy Dale

  • First timers
  • *
  • Posts: 4
    • View Profile
How do aeroplane wings work?
« Reply #2 on: 27/03/2011 19:22:08 »
Robert Bronson  asked the Naked Scientists:
   
Hello, I have just found your newbielink:http://www.thenakedscientists.com/HTML/podcasts/ [nonactive] this year and newbielink:http://www.thenakedscientists.com/HTML/podcasts/ [nonactive]

I have noticed, however, that on several occasions your team has described the flight of airplanes as being caused by the air pushing up on the wings. 

It was always explained to me that airplanes fly due to lift created by the vacuum caused on the top of the wing because of the curvature of the wing. 

I was told that it was similar physics to why sailboats work. Which is the correct answer?

Robert
Seattle, WA

What do you think?

Very little lift is directly caused by low pressure (the air pulling the wing up) and this is easy to prove with a simple experiment. Low pressure is a characterization of the wing and air having a tug of war. Will the wing pull the air down (Newton) or will the air pull the wing up (Burnulie) and how much?

The shape of the top of the wing does not cause low pressure so much as it causes the low pressure that is there to be efficiently pulled downward. Hold the curved back of a plastic spoon into the downward flow of water from the kitchen faucet; it takes a little force to get it out because the spoon is diverting some water away from the low pressure flow as a result of its curved shape (Newton). Now hold a plastic knife up to the flow at zero angle of attack and there is no noticeable force, although I am sure there is some that is the result of the low-pressure flow (Bernoulli). What the flow lacks in low pressure it makes up for in viscosity, this experiment works just as well with a less viscous air, hose. Even though the pressure differentials are the same one object generates a very noticeable amount of lift and one does not because of their difference in shape.

 

Offline Geezer

  • Neilep Level Member
  • ******
  • Posts: 8328
  • "Vive la résistance!"
    • View Profile
How do aeroplane wings work?
« Reply #3 on: 27/03/2011 21:11:22 »

Very little lift is directly caused by low pressure (the air pulling the wing up)


Strictly speaking, there is no "pulling" on the surface of the wing; there is only pushing. There is a pressure difference between the surfaces of the wing, and the high pressure side (the lower surface) exerts a greater force on the wing than the lower pressure side (the upper surface) exerts. The difference is the lift force.
 

Offline Roy Dale

  • First timers
  • *
  • Posts: 4
    • View Profile
How do aeroplane wings work?
« Reply #4 on: 28/03/2011 00:15:01 »

Very little lift is directly caused by low pressure (the air pulling the wing up)


Strictly speaking, there is no "pulling" on the surface of the wing; there is only pushing. There is a pressure difference between the surfaces of the wing, and the high pressure side (the lower surface) exerts a greater force on the wing than the lower pressure side (the upper surface) exerts. The difference is the lift force.

Areas of low pressure are areas where the wing and air are pulling on each other. Low pressure is a measurement of the wing and the air physically pulling on each other. Not only that but the air all around the wing is pulling on its surface due to the airs viscosity. So strictly speaking there is pulling on the wing in more ways than one.   
 

Offline yor_on

  • Naked Science Forum GOD!
  • *******
  • Posts: 11993
  • Thanked: 4 times
  • (Ah, yes:) *a table is always good to hide under*
    • View Profile
How do aeroplane wings work?
« Reply #5 on: 28/03/2011 06:46:35 »
A wing is a tricky subject. Take the upper side of that wing, build a air scoop on it, a little like the Harrier use to direct air downwards. Let the air follow the scoop and get directed to the ground. Now you have a 'action and a reaction'. The reaction is that your wing takes flight :)

Take away the air scoop but keep the wing, because, that's what the wing does. It directs the air downwards giving you a lift. That's also why you only see structures under a wing, building them over the wing will give that wing a very bad lifting coefficient. and how the wing diverts the air downwards? That's a result of the air molecules viscosity, they 'cling' to the wing, following its slope backwards and so gets redirected. The Bernoulli explanation is a often cited example of why, ah, wings take flight :) But it's not enough to explain it.

"The lift of a wing is equal to the change in momentum of the air it is diverting down. Momentum is the product of mass and velocity. The lift of a wing is proportional to the amount of air diverted down times the downward velocity of that air. Its that simple. (Here we have used an alternate form of Newton’s second law that relates the acceleration of an object to its mass and to the force on it; F=ma) For more lift the wing can either divert more air (mass) or increase its downward velocity. This downward velocity behind the wing is called "downwash"."
==

As Roy says..
« Last Edit: 28/03/2011 06:51:53 by yor_on »
 

Offline yor_on

  • Naked Science Forum GOD!
  • *******
  • Posts: 11993
  • Thanked: 4 times
  • (Ah, yes:) *a table is always good to hide under*
    • View Profile
How do aeroplane wings work?
« Reply #6 on: 28/03/2011 07:10:19 »
Which reminds me, anyone seen that question about a plane on a treadmill? The band moving backwards with the exact same speed as the plane tried to move forward, wanting to 'take of'? There was this enormous thread going on 'somewhere' filled with bright guys & gals, and some self pronounced 'physicists' :) ahem, explaining that it 'must' take flight as it was a SCIENTIFIC PRINCIPLE ::))

I loved baiting that treadmill :)

(and yes, the thread, & band, adapted to any change in speed from the plane, instantly, more or less)
 

Offline Geezer

  • Neilep Level Member
  • ******
  • Posts: 8328
  • "Vive la résistance!"
    • View Profile
How do aeroplane wings work?
« Reply #7 on: 28/03/2011 08:19:27 »

It directs the air downwards giving you a lift.


Not necessarily Yoron. An aerofoil can produce lift even with a zero degree angle of attack. In that situation, the lift is produced because the air flow across the top surface of the aerofoil is faster than the air flow across the bottom surface. This results in greater air pressure on the bottom surface than on the top, which creates an upward force on the wing.
 

Offline yor_on

  • Naked Science Forum GOD!
  • *******
  • Posts: 11993
  • Thanked: 4 times
  • (Ah, yes:) *a table is always good to hide under*
    • View Profile
How do aeroplane wings work?
« Reply #8 on: 28/03/2011 10:23:02 »
That too Geezer, But how about it?

Will the airplane take off?
 

Offline JMLCarter

  • Sr. Member
  • ****
  • Posts: 143
    • View Profile
How do aeroplane wings work?
« Reply #9 on: 28/03/2011 18:40:12 »
Idk, I think take-off usually uses high drag approach by lifting the nose to like 30° (you need a very long runway otherwise), but when it is cruising at 30000ft, the clear majority of the lift is due to wing curvature causing a pressure differential.
« Last Edit: 28/03/2011 18:41:44 by JMLCarter »
 

Offline Geezer

  • Neilep Level Member
  • ******
  • Posts: 8328
  • "Vive la résistance!"
    • View Profile
How do aeroplane wings work?
« Reply #10 on: 28/03/2011 18:45:51 »
That too Geezer, But how about it?

Will the airplane take off?

Probably not! The amount of lift produced by an aerofoil with a zero degree angle of attack is quite small. You have to vector a significant mass of air down to generate a lot of lift up.
 

Offline yor_on

  • Naked Science Forum GOD!
  • *******
  • Posts: 11993
  • Thanked: 4 times
  • (Ah, yes:) *a table is always good to hide under*
    • View Profile
How do aeroplane wings work?
« Reply #11 on: 28/03/2011 22:18:42 »
No, I mean the tread belt Gezzer..
Will that Areoplane lift or not?

I'm still searching for the ultimate truth, the universe and all there :)
 

Offline Roy Dale

  • First timers
  • *
  • Posts: 4
    • View Profile
How do aeroplane wings work?
« Reply #12 on: 29/03/2011 01:39:20 »

It directs the air downwards giving you a lift.


Not necessarily Yoron. An aerofoil can produce lift even with a zero degree angle of attack. In that situation, the lift is produced because the air flow across the top surface of the aerofoil is faster than the air flow across the bottom surface. This results in greater air pressure on the bottom surface than on the top, which creates an upward force on the wing.

The reason an airfoil can produce lift in the upward direction at zero angle of attack is because it diverts air downward as a result of its shape like I said before. One of the airfoil shapes that generates the most lift at zero angle of attack is the under cambered wing. The distance over the top of the under cambered wing is close to the distance under the bottom. The only thing better that shaping the top of the wing to divert the air downward is shaping the bottom to do the same.

Burnulie’s effect requires the motion of the air stream but the relative airflow that causes lift does not. Take an airplane that is flying through the air, the air that is generating its aerodynamic force is made up of still air. This air is not moving over the wing the wing is moving through the air. For this air to experience low pressure as a result of its increased speed it has to increase its speed. In order for it to increase speed it has to have speed. The way the wing sets this still air in motion is by pulling on it and pushing on it (measured in pressure). Some pull is from viscosity.  After this air has been set in motion its pressure may go down but it would not be in motion unless the wing pushed or pulled on it, and the air pushed and pulled back causing pressure.  You don’t have to wonder what came first the chicken or the egg or in this case the pressure or the motion of the air.
 

Offline yor_on

  • Naked Science Forum GOD!
  • *******
  • Posts: 11993
  • Thanked: 4 times
  • (Ah, yes:) *a table is always good to hide under*
    • View Profile
How do aeroplane wings work?
« Reply #13 on: 29/03/2011 03:18:15 »
I'm still hoping that someone can prove to me, once and for all, if that plane on the treadmill can lift or not?

I say it will not..

But..?

Awh, come on now, myth-busters has defined it as lifting, and the majority of that physics community I was on then seemed to think it should too?

==the exact quote;



A plane is standing on runway that can move (some sort of band conveyer). The plane moves in one direction, while the conveyer moves in the opposite direction. This conveyer has a control system that tracks the plane speed and tunes the speed of the conveyer to be exactly the same (but in opposite direction).

The question is:

Will the plane take off or not? Will it be able to run up and take off?

==end.

Jump right in, but prove your point sientiflicly please, I do have some standards?
I think?
 

Offline yor_on

  • Naked Science Forum GOD!
  • *******
  • Posts: 11993
  • Thanked: 4 times
  • (Ah, yes:) *a table is always good to hide under*
    • View Profile
How do aeroplane wings work?
« Reply #14 on: 29/03/2011 03:30:58 »
"Planes, however, accelerate a mass of air backwards. The equal and opposite reaction causes the plane to be accelerated forwards. A treadmill has no effect on the air being accelerated. It cannot deprive the plane of the equal and opposite reaction, and thus cannot prevent its acceleration.

But wait, doesn't the motion of the treadmill pull the wheels backwards with enough force to counteract the thrust of the engines? Simply put: No.

Watch planes at an airport. If the friction in the wheels were that significant, planes would be forced to use almost all of their available power just to taxi down the runway. Instead, planes use only a fraction of their available power to taxi."

QED.

(Sorry, started to read it again, I had some excellent discussions about it.)

Some weird facts; The conveyor thread was originally a Russian one from forum.ixbt.com Started by a guy calling himself shipwreck in 2003, and here is the same page translated by Google Google translation. and wandered in to our Western world July 19, 2005. And deeming of the reactions on my old site I would say a majority believed it able to 'take off'.

Sorry Robert, if you find me straying from the original question here, a very low attention threshold is mine I'm afraid. But I think you got the best answers we had.
« Last Edit: 29/03/2011 04:10:01 by yor_on »
 

Offline Geezer

  • Neilep Level Member
  • ******
  • Posts: 8328
  • "Vive la résistance!"
    • View Profile
How do aeroplane wings work?
« Reply #15 on: 29/03/2011 06:21:33 »

The reason an airfoil can produce lift in the upward direction at zero angle of attack is because it diverts air downward as a result of its shape like I said before. One of the airfoil shapes that generates the most lift at zero angle of attack is the under cambered wing. The distance over the top of the under cambered wing is close to the distance under the bottom. The only thing better that shaping the top of the wing to divert the air downward is shaping the bottom to do the same.

Burnulie’s effect requires the motion of the air stream but the relative airflow that causes lift does not. Take an airplane that is flying through the air, the air that is generating its aerodynamic force is made up of still air. This air is not moving over the wing the wing is moving through the air. For this air to experience low pressure as a result of its increased speed it has to increase its speed. In order for it to increase speed it has to have speed. The way the wing sets this still air in motion is by pulling on it and pushing on it (measured in pressure). Some pull is from viscosity.  After this air has been set in motion its pressure may go down but it would not be in motion unless the wing pushed or pulled on it, and the air pushed and pulled back causing pressure.  You don’t have to wonder what came first the chicken or the egg or in this case the pressure or the motion of the air.


So, basically you are saying that the traditional story about the Bernoulli effect is a load of cobblers slightly suspect  :D
 

Offline Roy Dale

  • First timers
  • *
  • Posts: 4
    • View Profile
How do aeroplane wings work?
« Reply #16 on: 29/03/2011 23:59:12 »

So, basically you are saying that the traditional story about the Bernoulli effect is a load of cobblers slightly suspect  :D
[/quote]


Yes as it pertains to wing lift production, I think the Bernoulli effect is a real and fascinating phenomena. 
 

The Naked Scientists Forum

How do aeroplane wings work?
« Reply #16 on: 29/03/2011 23:59:12 »

 

SMF 2.0.10 | SMF © 2015, Simple Machines
SMFAds for Free Forums